This talks about the changes to the Powerup Requirements API at PoGoAPI.net that have been made to handle level 40-50 power up.
Powering up Pokemon between level 40 and 50
To power a Pokemon above level 40 you now need to collect and use XL candy. This is gained from a variety of means including catching evolved or high level Pokemon. Similar to normal candy, XL candy is per species so you will need to collect it for the Pokemon species you wish to power up.
Another way to get additional XL candy is to use a XL rare candy to give you one XL candy of a chosen Pokemon species.
Changes to the API
To handle the fact that XL candy is used at later levels the new key xl_candy_to_upgrade has been added to allow you to access how much XL candy is needed at each level. This will be 0 for levels 1 to 40 and an integer for levels 40 to 50. An example of these changes are below.